There’s something about the Enlightenment. Today, few educated men and women spend much time debating whether Western civilization took a disastrously wrong turn in the High Middle Ages. They do not blame all manner of political ills on Romanticism, or insist that non-Western immigrants adopt Renaissance values. But the Enlightenment is different. It has been held responsible for everything from the American Constitution to the Holocaust. It has been defended as the birthplace of human rights and condemned as intolerant, cold, abstract, imperialist, racist, misogynist, and anti-religious. Edmund Burke, in one of the most eloquent early attacks, excoriated “this new conquering empire of light and reason.” One hundred fifty years later, Max Horkheimer and Theodor Adorno declared bluntly that “enlightenment is totalitarian.” It remains very hard to shake the sense that something fundamental about the modern world owes its form to the intellectual revolutions of eighteenth-century Europe—and that to repair the world, we may need to start by identifying what those revolutions got wrong.

As in all such intellectual controversies, misunderstandings abound, for no one can even agree on what is being argued about. Everything about “the Enlightenment”—its unity, its goals, its methods, its chronological and geographical boundaries—remains in dispute, providing activity, if not always gainful employment, for a sizeable academic industry. While some scholars limit the Enlightenment to a small cadre of eighteenth-century philosophes, others identify it with tendencies in Western thought that may stretch all the way back to classical antiquity. The title phrase of Kant’s famous essay “What Is Enlightenment?” is a question that has launched a thousand—ten thousand!—dissertations....

OVER THE PAST decade ... there has appeared a ... champion of a united and beneficial Enlightenment: Jonathan I. Israel, a distinguished English historian who first made his reputation working on the early modern Netherlands. Democratic Enlightenment is the third of three massive volumes, totaling nearly three thousand pages. Whereas [Ernst] Cassirer and [Peter] Gay and other scholars limited themselves mostly to the eighteenth century, Israel burrows back deep into the seventeenth century. Whereas Cassirer and Gay and other scholars concentrated on France, Germany, and Scotland, Israel’s first volume gave pride of place to the Netherlands, and this most recent one soars over a dazzling variety of landscapes. Like Cassirer and Gay, Israel sees a fundamental tension at the heart of the Enlightenment, in his case between the tendencies that he labels “radical” and “moderate.” He goes so far as to speak of “two consciously opposed and rival enlightenments.” But unlike Cassirer and Gay, he is no dialectician. For him, the only truly significant Enlightenment was the radical one....